1. Field of the Invention
The present invention relates to a novel method of measuring particles in a liquid sample. The invention also relates to a system for measuring particles in a liquid sample. The invention allows for effective suppression of bubbles in liquid samples, and has particular applicability in the semiconductor and pharmaceutical manufacturing industries.
2. Description of the Related Art
In many industries, such as the semiconductor and pharmaceutical manufacturing industries, it is important that the process chemicals employed have very low particle concentrations. Typical chemicals employed in the semiconductor and pharmaceutical manufacturing industries include, for example, ammonium hydroxide, hydrogen peroxide, sodium hydroxide, hydrofluoric acid, nitric acid, sulfuric acid, phosphoric acid, acetic acid, isopropyl alcohol, tetramethylammonium hydroxide, de-ionized water and mixtures thereof. It is desirable that concentrations of less than 10 particles (&gt;0.2 .mu.m) per milliliter be maintained in such chemicals.
The particles present in the chemicals may be present in solid form, in colloid form or in a combination of those two forms. In any case, such particles are harmful to the manufacturing processes and the product formed, and bring about a reduction in the product yield.
Particles in liquids, or liquid-born particles, are conventionally monitored for quality control and production assurance with an optical-type particle counter or monitor. In such instruments, a sample of the liquid laden with particles is illuminated by a laser beam and light is scattered by the particles. The scattered light is collected by the instrument, and particle counts and size information is generated based on the scattered light measurements.
In addition to the particles in the liquid chemicals, micro-bubbles are also present. The micro-bubbles can be attached to the particles or can be freely dispersed in the liquid. In contrast to the particles in solution, micro-bubbles are not always detrimental to the products being formed.
It has been found that during particle measurement with optical-type particle instruments, micro-bubbles exhibit similar light scattering characteristic to particles. As a result, optical-type particle instruments cannot distinguish between the bubbles and the particles, and bubbles will be counted as particles. Chemicals which are actually within process specifications may, therefore, be wasted due to the artificially high particle counts resulting from the mischaracterization of micro-bubbles as particles.
A countermeasure which has been proposed for eliminating the effects of micro-bubbles on liquid-born particle measurements is suppression of the micro-bubbles by pressure. Optical-type particle counters having a compression chamber are commercially available, for example, the Particle Measuring Systems Inc. CLS-700 and the HIAC/ROYCO 8000A systems. In such systems, compressed air or nitrogen is applied directly to the liquid surface. This is intended to compress the gas in the bubbles back into the liquid.
It has, however, been found that while relatively large bubbles can be suppressed in this manner, a multitude of smaller bubbles are formed by the break-up of the larger bubbles and/or by a mere reduction in size of the larger bubbles. In addition, bubbles formed by the chemical's decomposition gases, regardless of size, cannot effectively be compressed back into the liquid.
Another method for dealing with the presence of micro-bubbles is disclosed in U.S. Pat. No. 4,783,599, to Borden. In that patent a system is described which distinguishes non-contaminant bubbles from contaminant particles. The substantially spherical bubbles are detected by symmetrically spaced photodiodes. The irregularly shaped contaminant particles are detected and the presence of the bubbles are negated by the detection system. This method, however, may be limited to large bubbles and irregularly shaped particles. Spherical particles would necessarily be mischaracterized as bubbles.
To meet the requirements of the semiconductor and pharmaceutical manufacturing industries and to overcome the disadvantages of the related art, it is an object of the present invention to provide a novel method of measuring particles in a liquid sample.
It is also an object of the present invention to provide a system for measuring particles in a liquid sample, with which the inventive method can be practiced.
The method and apparatus in accordance with the invention allow for the effective suppression of bubbles present in a liquid chemical sample, thereby allowing for accurate particle measurements.
Other objects and advantages of the present invention will become apparent to one of ordinary skill in the art upon review of the specification, drawings and claims appended hereto.